In conventional planographic printing, a printing plate bearing as oleophilic, ink receptive image is first dampened with an aqueous fountain solution to prevent ink from wetting the hydrophilic, non-image bearing areas of the printing plate, after which an oil-based ink is rolled over the plate to selectively coat the now printable image. Conventional planographic printing has some difficulties inherent in having both an oleophilic ink and an aqueous fountain solution conjoined in the same press. First, the fountain solution applied to the printing plate flows back into the train of inking rollers on the press, causing emulsification of the ink. Secondly, it is difficult to maintain control of the delicate balance needed between the amount of ink and the amount of fountain solution applied to the printing plate. Consequently, the image fidelity and uniformity are difficult to maintain. Thirdly, the fountain solution tends to flow forward over the offset cylinder, moistening the copy paper and thereby causing its dimensional change. Fourthly, in the case where printing is imaged directly by electrophotography, the imaged printing plate must be subjected to an etching treatment and the printing operation becomes complicated.
Considerable effort has been applied in the industry directed toward the development of lithographic printing plates that may overcome some of the foregoing problems. A significant portion of that effort has been directed toward the development of planographic plates that do not need a fountain solution circulating in the printing apparatus to accomplish the printing function. These plates are referred to herein as waterless plates or dry plates. For these waterless plates, the circulating fountain solution is avoided by the discovery of various printing methods and plate compositions that do not rely on the induced hydrophilicity of a portion of the plate to distinguish an oleophilic image surface from a non-oleophilic non-image surface.
U.S. Pat. No. 4,259,905 teaches a waterless, contact speed planographic printing plate having an overlaying modified organopolysiloxane polymeric material layer. The plate exhibits enhanced printing endurance and produces prints of low background contamination.
U.S. Pat. No. 4,342,820 teaches a negative working waterless plate requiring no dampening water for use in negative work which comprises a base substrate, a light releasing photosensitive layer overlaying the base substrate and the silicone rubber layer overlaying the photosensitive layer. When the printing master plate is exposed through a negative film and then treated with a developer, only the silicone rubber layer overlaying the exposed photosensitive layer is removed, while the photosensitive layer remains as it is to form and image area. Dampening water is not required when printing is carried out.
U.S. Pat. No. 3,894,873 teaches a positive working waterless plate, comprises a base substrate, a light sensitive photoadhesive layer overlaying a base substrate and the silicone rubber layer overlaying the photoadhesive layer. When the printing master plate is exposed through a positive transparency and then treated with a developer, only the silicone rubber layer overlaying the unexposed photoadhesive layer is removed, while the photoadhesive layer remains as it is form an image area.
The waterless plates described in the foregoing patents are contact speed and not for camera speed. The term "camera speed` indicates that the light-sensitive material in question is photographically sensitive enough to permit imagewise exposure by focusing an original onto the light-sensitive layer of the printing plate forming material through an image-focusing lens system or other high sensitivity or digitized imaging devices. On the other hand, the term "contact speed" means that a light-sensitive material has a sensitivity such that it can be exposed through a negative or positive transparency placed in contact with it. Camera speed materials are photograpically sensitive to relatively low energy levels of light while relatively high light energy levels are needed to expose contact speed materials.
Most light-sensitive lithographic printing plate materials presently in commercial use are of contact speed sensitivity. Thus an image-bearing transparency of the original must be prepared before a printing plate can be made from a contact speed material. Consequently, the number of steps necessary to prepare the lithographic printing plates is considerably more than in the case in which no transparency is necessary.
The sensitizers typically used in light-sensitive lithographic printing plate materials are diazo compounds, i.e., o-quinonediazide compounds, or light-sensitive polymers like polyvinyl cinnamates. However, the photographic speed of these materials is too slow for the materials sensitized therewith to be of camera speed. Therein lies the reason for the preceding statement to the effect that most light sensitive lithographic printing plate making materials are of contact speed.
The most common sensitizer used in photographic materials of camera speed is silver halide. Light-sensitive lithographic printing plate materials having solely a silver halide emulsion layer as the image-forming layer have been produced. However, such plate materials are comparatively difficult to process and the printing life thereof in not long. Other light-sensitive lithographic printing plate composite materials of camera speed are known.
U.S. Pat. No. 3,567,445 teaches a presensitized lithographic printing plate with two different spectrally sensitive layers separated by a Novolak resin. The base layer is of the type used in a negative-working presensitized lithographic plate while the top layer is a silver halide emulsion layer.
U.S. Pat. No. 4,229,912 discloses a process for producing a lithographic printing plate using a plate containing two light sensitive layers, one of which is a gelatino-silver halide emulsion and the other is a non-silver halide layer. The patent teaches the use of a proteolytic enzyme to aid in the removal of the gelatino layer after imagewise exposure followed by exposure of the non-silver halide layer.
U.S. Pat. No. 4,283,478 teaches a light-sensitive printing plate forming material comprising a support having a hydrophilic surface provided with a layer of a positive-working light-sensitive resin composition capable of forming an oleophilic image, and a tannable light-sensitive silver halide emulsion layer containing a tanning-developing agent.
U.S. Pat. No. 4,341,856 teaches printing plates containing at least two light sensitive layers. Production of the final image-containing printing plate requires at least two exposure steps.
U.S. Pat. No. 5,330,875, incorporated herein by reference, teaches a process for producing negative and positive original images on a bilevel printing plate. The plate utilizes a contact speed (positive or negative) layer and a camera speed (negative) overlayer in a single development process to form a temporary imagemask which is exposed sequentially to either the negative or positive original with the resultant image subsequently developable in a single process or step.
The objective of the present invention is to provide a camera speed waterless lithographic printing plate wherein the need for conventional dampening with an aqueous fountain solution in the printing operation is obviated.
A further objective of the invention relates to the presentation of a waterless lithographic printing plate amenable to the production of positive originals using digital imaging devices in a single development process.
Yet further objectives of the invention include the method for the production of the waterless plate as well as the methods for image formation employing the waterless plate.